A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Fuel cells are a clean and efficient power source, and may replace traditional power sources such as the internal combustion engine in both stationary and automotive power applications.
In a polymer electrolyte membrane (PEM) fuel cell, the electrolyte is a solid polymer membrane which is electronically insulating but ionically-conducting. Proton-conducting membranes such as those based on perfluorosulphonic acid materials are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.
The principle component of a polymer electrolyte fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer membrane. On either side of the membrane there is an electrocatalyst layer, typically comprising a platinum-based electrocatalyst. An electrocatalyst is a catalyst that promotes the rate of an electrochemical reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion substrate. The gas diffusion substrate must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the substrate must be porous and electrically conducting.
In many practical fuel cell systems hydrogen fuel is produced by converting a hydrocarbon fuel (such as methane or gasoline) or an oxygenated hydrocarbon fuel (such as methanol) to a gas stream known as reformate in a process known as reforming. The reformate gas contains hydrogen, about 25% carbon dioxide, small amounts of carbon monoxide (typically at levels of around 1%) and may contain other contaminants. For fuel cell operating temperatures below 200° C. and especially for PEM fuel cells operating at temperatures of around 100° C., carbon monoxide, even at levels of 1-10 ppm, is a severe poison for the platinum electrocatalyst in the anodes of the MEAs. This leads to a significant reduction in fuel cell performance (i.e. the cell voltage at a given current density is reduced).
To alleviate anode carbon monoxide poisoning, most reformer systems include an additional catalytic reactor known as a preferential or selective oxidation reactor. Air or oxygen is injected into the reformate gas stream, which is then passed over a selective oxidation catalyst which oxidises the carbon monoxide to carbon dioxide. This can reduce the levels of CO from about 1% down to below 100 ppm, but even at these levels the anode electrocatalyst is poisoned.
WO 00/35037 discloses carbon monoxide tolerant anodes comprising a first electrocatalyst of formula Pt—Y, wherein Y is a bronze forming element, and a second electrocatalyst of formula Pt—M, wherein M is a metal alloyed with the platinum, wherein the first and second electrocatalysts are in ionic contact. The first and second electrocatalysts may be formulated into two separate layers, which are applied to the one side of the gas diffusion substrate or to the membrane. Alternatively, the first and second electrocatalysts may be mixed together and formed into one layer containing both catalysts. Membrane electrode assemblies comprising the anodes exhibit good performance even at levels of 100 ppm CO.